This invention is directed to attaining higher data compression ratios in block based image compression schemes; and in particular, to attaining such ratios without reliance on color differences, and with little or no increase in data processing time.
In current video processing schemes, for example, a frame representing an image of video is compared against a reference frame. If the frame is essentially identical to the reference frame (i.e., the image obtained at a later point in time is essentially identical to the image obtained at an earlier point in time and represented by the reference frame), the information contained in the later acquired frame is discarded. Processing schemes incorporate a set of criteria (e.g., intensity threshold) by which the one frame is compared against the other, and only if the criteria are met, is information from the later acquired frame retained.
Further, even if the later acquired frame does meet the retention criteria, it is not necessarily desirable to retain all of the information contained in the frame. Rather, given system processing constraints and memory capabilities, it is usually desirable to only retain that portion of the frame containing information differing from that in the reference frame. The frames are typically divided into blocks of data and as part of the processing; various of the blocks are identified as those whose contents are to be saved. These blocks are referred to as macro blocks and if they are to be saved, they are “flagged” as part of the image processing. It often occurs that within an area of the image where blocks have been flagged, there are other macro blocks where the criteria threshold has not been met, even though the “unflagged” block represents a changed portion of the image. These blocks are referred to as “orphan” blocks. The problem with orphan blocks is that if the video imagery represented therein is not saved, holes or gaps will appear in the saved image when the stored information is subsequently recalled and displayed.
Orphan blocks are not particularly noticeable in black and white (B&W) images because the intensities of the blocks comprising the reference frame are very close to those in the frame being compared. In color images, the failure to save orphan blocks is very noticeable, particularly where the colors are different, but the intensities are still relatively close. A typical result of this type situation would be where a portion of a background in front of which a person is standing appears in the middle of the image of a person's face. A potential solution to this problem would be to investigate color within the image. However, to do so requires examining every pixel within the macro block and this would require substantial additional processing time. The present invention employs a process involving intensity comparisons and identifying as orphaned macro blocks those surrounded by flagged macro blocks.